phase2: fix invalid escape sequence flake8 warning in checksums step

phase2: fix invalid escape sequence flake8 warning in checksums step

Resolved a flake8 linting warning related to an invalid escape sequence
in the ShellCommand for calculating checksums:

 phase2/master.cfg:739:28: W605 invalid escape sequence '\('
 phase2/master.cfg:739:32: W605 invalid escape sequence '\)'
 phase2/master.cfg:739:35: W605 invalid escape sequence '\('
 phase2/master.cfg:739:39: W605 invalid escape sequence '\)'

The warning was caused by the use of unescaped parentheses in a regular
expression within a sed command.

Use a raw string (with an 'r' prefix) to treat backslashes as literal
characters, ensuring that the regular expression is correctly
interpreted and flake8 does not raise a warning.

This fix ensures that the code adheres to Python's string handling best
practices and maintains the integrity of the regular expression
functionality.

Fixes: f0faed2970dd ("phase2: compute checksums")
Signed-off-by: Petr Štetiar <ynezz@true.cz>